Method and system for processing doppler signal gaps

ABSTRACT

A method and system for processing Doppler signal gaps is disclosed. The method comprising the steps of: receiving Doppler signals during Doppler signal acquisition to obtain data of in-phase (I) component and quadrate (Q) component signals of the Doppler signals; receiving interruption of the Doppler signals; estimating the data of the Doppler signals interrupted in said gap period filling the interrupted Doppler signals with the data of the estimated Doppler signals so that the data of the I and Q component signals in said gap period are filled to form a continuous output; and generating Doppler spectrum data or Doppler sound based on the data of the filled I and Q component signals.

FIELD OF THE INVENTION

The present invention relates to ultrasonic techniques, and inparticular to data processing in a multimode ultrasonic scanning systemand a method for processing Doppler signal data.

BACKGROUND OF THE INVENTION

Modern medical ultrasonic diagnostic systems usually can combinedetecting contents and display them simultaneously, for example,synchronously displaying two-dimensional B mode image and Dopplerspectrum diagram, or synchronously displaying Doppler spectrum diagramand colour blood flow image, by which a doctor diagnoses diseases.

FIG. 1 shows a typical medical ultrasonic imaging system for measuringDoppler blood flow velocity. The system comprises a multi-elementultrasonic transducer array (not shown), which is capable of convertinghigh voltage electrical pulses into ultrasonic waves in transmittingstages and converting ultrasonic echoes into electric signals inreceiving stages. The echo signals received by each of the array unitsof the transducer are sent to a beamformer module, and are processed toimprove the signal-to-noise ratio of the echo signals. Then, accordingto the characteristics of the echoes, the system sends output signals ofsaid beamformer to respective signal processing modules, wherein thoserelated to the B mode images are sent to a two-dimensional B mode signalprocessing module for obtaining two-dimensional B mode image data; thoserelated to spectral Doppler information are sent to the spectral Dopplersignal processing module for obtaining Doppler spectral data; and thoserelated to colour blood flow are sent to the colour blood flow signalprocessing module for obtaining colour blood flow data. Finally, adisplay module combines said two-dimensional B mode image data, Dopplerspectral data and colour blood flow data to form resultant data forsynchronously displaying on the display screen.

FIG. 3 shows a flowchart of processing Doppler spectral signals, forexample, in an ultrasonic Doppler blood flow analysis system. After thebeamforming of the ultrasonic echo, an RF echo is formed, which isdecomposed into two component signals by a demodulation module, i.e., anin-phase component I signal and a quadrate component Q signal. Then, ina continuous wave Doppler system, the I and Q components are directly inwall filtering processing stage; in a pulsed wave Doppler system, the Iand Q components are gated in range first, respectively, that is,accumulated in a specific time interval, the accumulation time intervaland the length of the pulsed Doppler transmitted pulse are selected byan operator according to actual situations, then the I and Q componentsare in the wall filleting processing stage. The wall filter is a highpass filter, and can filter clutters caused by stable or slow movingtissues. The I and Q components after the processing of this stage,which mainly comprise the echoes caused by the motion of red bloodcells, are sent to a power spectrum estimation module, which estimates apower spectrum usually by the use of fast Fourier transform (FFT). Thenumber of points of the FFT may be 128 or 256. Since the dynamic rangeof the estimated power spectrum is too wide, it is necessary for theestimated power spectrum to be compressed into a gray scale displayrange. The Doppler spectrum diagram finally displayed on the screenrepresents the power spectral intensity at a certain time and at acertain velocity, i.e. at a certain frequency shift. The system mayfurther comprise an automatic envelope detection module for analyzingthe compressed data, automatically tracking the variations of the peakvelocity and mean velocity of the blood flow with time, and displayingthem on the Doppler spectrum diagram in real time. Furthermore, thewall-filtered I and Q data may further be sent to an acoustic processingmodule to form acoustic data of the forward blood flow and the reverseblood flow, then these data are D/A converted and sent to a speaker,respectively, so as to generate sounds of forward and reverse bloodflows.

FIG. 2 shows a two-dimensional B type image and a spectral Dopplerdiagram synchronously displayed by the system. The upper portion of thefigure indicates the two-dimensional B type image in which the dottedlines show the positions and directions of blood vessels. The operatormay select sampling lines corresponding to the blood flow to be detectedand corresponding interested regions. The lower portion of the figureindicates the Doppler spectrum diagram of blood flow in the selectedregion.

In order to display synchronously the two-dimensional B mode image andspectral Doppler image on the screen, as shown in FIG. 2, the ultrasonicimaging system usually performs fast switching between thetwo-dimensional B mode scanning and Doppler scanning. Thus, the B modeimage scanning and the Doppler blood flow measurement scanning areperformed in different time intervals. The fast switching between twodifferent scanning modes has the advantage that the interaction of theimaging results is very small, because the scanning of the two systemsare performed in different time intervals; the two systems can share asingle scanhead; and the Doppler transmitting mode may be a pulsed or acontinuous mode. But there is also an inherent drawback in this manner,that is, the Doppler signal may be missed due to interrupt of theDoppler scanning while performing B type image scanning. The update oftwo-dimensional B type image and Doppler diagram implemented in thismanner must result in discontinuity in the Doppler spectral diagram andDoppler sound. The discontinuous time interval of the Doppler signalcaused by the switching to other working modes are called a gap.

Referring to FIG. 2, the solid lines correspond to the spectrum diagramin non-gap time intervals, and the dotted lines correspond to thespectrum diagram in gap time intervals. There is no Doppler signal inthe gap time intervals; and the Doppler spectrum diagram is interrupted,that is, there is no Doppler spectrum in the time intervals of thedotted lines. In the gap time intervals, the Doppler sound is alsointerrupted due to lack of Doppler signals. Therefore, in a multimodeultrasonic scanning synchronous display system, a gap filling method isusually used to compensate visual or audio discontinuity of the Dopplerspectrum diagram or Doppler sound caused by the gaps. As illustrated bythe dotted line blocks and the flowchart in FIG. 3, the function of thegap filling module is to estimate the lost Doppler signals and to makethe Doppler signals continuous, thereby the continuity of the Dopplerspectrum diagram and the Doppler sound can be maintained.

In the technical solution disclosed in U.S. Pat. No. 5,476,097, Robinsonproposed a method of filling the gaps of the results of the Dopplerspectrum diagram and the Doppler sound after Doppler processing, so asto make them more continuous on visual or audio effect. The gaps in theDoppler spectrum diagram are filled by the use of a spanning method;each of the calculated spectra near the gaps is repeated for two times;and the gaps of the Doppler sound are filled by the use of directlyrepeating the Doppler sound results in the non-gap intervals.

In the technical solution disclosed in U.S. Pat. No. 4,559,952, Angelsenproposed a method of directly filling I and Q Doppler signals. Accordingto that method, the I and Q Doppler signals before the gap are repeatedin the gap period, the I and Q Doppler signals are continuous in thesucceeding Doppler processing.

The above-mentioned prior arts have some disadvantages. The effect ofsound filling of the filling method of U.S. Pat. No. 5,476,097 isrelatively ideal, no interrupt can be felt audibly; however, the visualeffect of the Doppler Spectrum diagram filling is not satisfactory. Theeffect will be better where the difference of Doppler spectrum diagramsbefore and after the gap is not great. However when the difference ofthe spectrum diagrams before and after the gap is relatively great, asignificant sudden change can be seen at the joints of spanning due tothe use of the spanning method. The filling method of U.S. Pat. No.4,559,952 only employs the I and Q Doppler signals for filling; the gapinterval may not be too long, in order to result in a good fillingeffect. However, a too short gap interval may restrict the imagingquality of scanning of other modes.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome the disadvantages ofthe above mentioned prior arts and to provide a gap processing methodcapable of preferably filling the gaps of the Doppler spectrum diagramor the Doppler sound, and providing visual or audio continuity, withoutimpairing the imaging quality of other scanning modes of the ultrasonicscanning system.

In order to achieve the above-mentioned object, the present inventionprovides a method for processing Doppler signal gaps, comprising thesteps of:

receiving Doppler signals during Doppler signal acquisition to obtaindata of in-phase (I) component and quadrate (Q) component signals of theDoppler signals; receiving interruption of the Doppler signalsindicating that the output of the data of said I and Q component signalsis interrupted in a gap period; estimating the data of the Dopplersignals interrupted in said gap period based on the Doppler signalsreceived during Doppler signal acquisition filling the interruptedDoppler signals with the data of the estimated Doppler signals so thatthe data of the I and Q component signals in said gap period are filledto form a continuous output; and generating Doppler spectrum data orDoppler sound based on the data of the filled I and Q component signals.

In the filling step, the whole gap may be divided into at least two timeintervals; and the data of the interrupted Doppler signal in differenttime intervals may be filled with the data of Doppler signals before andafter the gap, respectively.

In the filling step, a predetermined filling data may be repeatedlyoutput in a predetermined gap time interval to be filled.

The time interval to which the predetermined filling data belongs may beshorter than the time interval to be filled, so that said predeterminedfilling data are repeatedly output more than once in said time intervalto be filled.

The filling step may further comprise weighting the data of the Dopplersignals so as to smooth the Doppler data in beginning, intermediate andend time intervals of said gap, caused by the repetition of thepredetermined filling data.

The weighting step may comprise processing the data of the Dopplersignals by a window function.

The method according to the present invention may further comprise:filtering the received Doppler signals by an initialized high passfilter to reduce the transient response time of said high pass filter,so as to shorten the gap period. The high pass filter may include asecond-order IIR filter.

The present invention provides an ultrasonic scanning synchronousdisplay system, comprising:

means for receiving Doppler signals during Doppler signal acquisition toobtain data of in-phase (I) component and quadrate (Q) component signalsof the Doppler signals; means for receiving interruption of the Dopplersignals, in which the output of the data of said I and Q componentsignals is interrupted and in a gap period; means for estimating thedata of the Doppler signals interrupted in said gap period based on theDoppler signals received during Doppler signal acquisition means forfilling the interrupted Doppler signals with the data of the estimatedDoppler signals so that the data of the I and Q component signals insaid gap period are filled to form a continuous output; and means forgenerating Doppler spectrum data or Doppler sound based on the data ofthe filled I and Q component signals.

By the use of the above-mentioned method and system, more smoothlycontinuous the Doppler signals can be provided; and further processingperformed on the filled continuous Doppler signals can provide morecontinuous Doppler spectrum image and Doppler sound, so that themultimode ultrasonic scanning system can realize synchronous display inlow cost, even the image quality of other non-Doppler imaging can beimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the flowchart of signal processing in an ultrasonic imagingsystem;

FIG. 2 shows the synchronous display of a two-dimensional B type imageand a spectral Doppler diagram;

FIG. 3 shows the flowchart of Doppler signal processing;

FIG. 4 shows the generation and analysis of a gap;

FIG. 5 shows the flowchart of the gap filling method of the presentinvention;

FIG. 6 shows the gap filling according to the method of the presentinvention; and

FIG. 7 shows a state-transition diagram of a second order IIR filter;and

FIG. 8 shows the synchronous display of the two-dimensional B type imageand spectral Doppler image after gap filling.

DETAILED DESCRIPTION OF THE INVENTION

In an embodiment of the present invention, a method for processingDoppler signal gaps is used in the Doppler spectral signal processingprocedure when the Doppler scanning is rapidly switched in an ultrasonicscanning synchronous display system, the method comprises followingsteps:

A. when the system operates in the Doppler scanning mode, data of the Iand Q component signals of the Doppler signal are obtained after thedemodulation, filtering, and A/D conversion of RF ultrasonic echoes,which are provided to the system for performing power spectralprocessing to generate the Doppler spectrum image data, or forperforming acoustic processing to generate Doppler sound;

B. the reception of the Doppler signals is interrupted when the systemoperates in non-Doppler scanning mode or in the initial stage ofreentering the Doppler scanning mode; the output of the data of said Iand Q component signals is interrupted and is in a gap period;

C. the Doppler signals interrupted in said gap period are estimated andfilled by the system based on the Doppler signals in the non-gap period,so that the I and Q component signals data in said gap period are filledto form a continuous output; and

D. the system generates the Doppler Spectrogram data or Doppler soundbased on the I and Q component signals data continuously outputted afterthe filling processing.

In particular, in said step C, the whole gap is divided into at leasttwo time intervals by the system, and the data of interrupted Dopplersignal in different time intervals are filled with the data of Dopplersignal before or after the gap, respectively.

In another embodiment of the present invention, when reenterring theDoppler scanning mode, the system reduces the transient response time ofan high pass filter by initializing the high pass filter, thereby partof the gap period corresponding to the initial stage of said Dopplerscanning in said step B is shortened.

FIG. 4 shows the generation of gaps, and takes the synchronouslydisplaying of two-dimensional B type image and spectral Doppler imagefor example. In the figure, the curve represents a Doppler spectrumdiagram, D represents that transmitting pulse correlates to Dopplerspectrum, and B represents that transmitting pulse correlates to two-dimensional B type image. As mentioned in the foregoing, the B typeimage scanning and Doppler blood flow velocity measurement scanning areperformed in different time intervals, the B type scanning and theDoppler scanning are switched from one to another rapidly; during the Btype scanning, the gaps caused by the missing of Doppler signal due tothe stop of scanning are referred to as “gaps caused by switching toother scanning modes”; when the transmitting pulse switches from B to D(as shown in the flowchart of FIG. 3), because the wall filter is a highpass filter (a transient state exists), when the Doppler signal is wallfiltered, the transient state of the wall filter may cause the Dopplersignals of the initially transmitted pulses D invalid and unable to beused, thereby resulting in occurrence of the gaps, which are referred toas “gaps caused by transient state”. Therefore, according to the presentinvention, the above mentioned technical problem may be resolved by theuse of two different schemes with respect to different originations ofgaps. The first scheme is that the missing I and Q Doppler data during agap are estimated simultaneously based on the Doppler data before andafter the gap, and the Doppler signals during the gap are filledtherewith, that is, the whole gap is divided into two time intervals,the gap Doppler data in the first time interval are the repetition ofthe Doppler data before the gap, while the gap Doppler data in thesecond time interval are the repetition of the Doppler data after thegap, thereby it is possible to make the Doppler signal more continuous;the second scheme is that a method of initializing the filter is used toestimate the Doppler signal prior to high pass filtering based on theDoppler signal after the gap, so that the transient effect of the highpass filtering can be reduced, thus the gap caused by the transientstate can be reduce to make the Doppler signal more continuous, andparticularly, a longer scanning time is permitted for other scanningmode to take. Thereby, the continuity of the Doppler spectrum diagram orDoppler sound can be maintained visually or audibly when a rapidswitching between the Doppler scanning and scanning of other modesoccurs.

FIG. 6 shows an embodiment of the gap processing method according to thepresent invention, in the filling step, the whole gap is divided by thesystem into at least two time intervals, and the data of Doppler signalinterrupted in different time intervals are filled with the data ofDoppler signals before and after the gap, respectively. FIG. 6illustrates an example of processing the I or Q Doppler data, the wholegap time interval comprises gaps caused by other scanning modes and thegap caused by wall filtering, and may be divided into two time intervals(including but not limited to the evenly divided time intervals shown inFIG. 6). The Doppler data in the first time interval can be estimated bythe use of the Doppler data before the gap, while the Doppler data inthe second gap time interval can be estimated by the use of the Dopplerdata after the gap. Various methods can be used as filling algorithm,wherein a simple one is the data repetition method, that is,predetermined filling data are repeatedly outputted in the predeterminedfilling gap time interval, for example, the Doppler data in the formergap are the repetition of the Doppler data before the gap, while theDoppler data in the latter gap are the repetition of the Doppler dataafter the gap. In addition, said predetermined filling data may berepeatedly outputted more than once in said time interval to be filled.Thus non-smooth and non-continuation may occur in the Doppler dataduring the beginning, intermediate and end time intervals of the gap. Bythe use of a weighting method, the system may smoothen and continuewhere the non-smooth and non- continuation occurs, for example, bygradually converging the Doppler data before the discontinuous point tozero, and gradually increasing the Doppler data after the discontinuouspoint from zero. Thereby, the continuity of the Doppler data can bemaintained. Taking the end of the gap in FIG. 6 as an example, theDoppler data before and after this point are weighted by the use of awindow function, respectively, so that continuity thereat of the Dopplerdata can be maintained.

The second embodiment of the gap processing method according to thepresent invention is shown in FIG. 5. When the transmitting mode is theDoppler scanning mode, the demodulated I and Q Doppler data inputtedinto the gap filling module are effective. These two data are processedby high pass filtering. The said high pass filter has the same filteringcharacteristics as that of the wall filter, and can filter off theclutters caused by the static or lowly moving tissues, thereby the gapfilling data are data of effective signals. When the high pass filterhas the same characteristics as that of said wall filter (which mayfurther replace the succeeding wall filtering processing in FIG. 3), thetransient response gap shown in FIG. 4 may be caused each time thescanning mode is switched from other scanning mode to the Dopplerscanning mode. If the high pass filter is being in a non-transientstage, then the two filtered Doppler data are sent to buffers,respectively, to be stored, and are outputted at the same time aseffective data, which is a part of the continuous data after the gapfilling processing; and if the high pass filter is being in thetransient stage, then said two Doppler data are ineffective, and the Iand Q data previously stored in the buffers are necessary to be used forgap filling to obtain continuous I and Q Doppler data. When thetransmitting mode is one of the other non-Doppler modes, then the I andQ Doppler data stored in the buffers are used to estimate the missing Iand Q signals in the gap.

The transient response time is generally relevant to a specific filter.When the high pass filter has a structure of an IIR filter, thetransient response time may be as long as ten times of the number oforders of the IIR filter. In order to reduce the length of the gap, whenthe system according to the present embodiment reenters the Dopplerscanning mode, a method of initializing the filter can be employed bythe system to reduce the transient response time, and thereby the gaptime caused by the transient response during initial stage of theDoppler scanning can be shortened. Thus the time of non-Doppler scanningcan be increased, and the quality of the images of other non-Dopplerimaging can be improved.

In this embodiment, the filling data used to fill the gaps in thetransient stage and non-Doppler scanning modes, may be the data storedin the buffers before the end of the Doppler scanning mode, or incombination with the embodiment as shown in FIG. 6, the gap is dividedinto two time intervals and filled with the data buffered before the endof the Doppler scanning mode and the data buffered in the non-transientstate at the beginning of next Doppler scanning mode, respectively.Substantially, the I and Q Doppler data after the gap filling of thepresent embodiment are continuously sent to subsequent modules forprocessing, including power spectrum estimation and acoustic processing,which are not described again.

The initialization of the filter means the estimation of the Dopplerdata before the signal by the use of the data of Doppler signal.Assuming that a second order filter is designed, the input is x(n), andthe output is y(n), then the filter can be expressed by:

y(n)=b ₀ x(n)+b ₁ x(n-1)+b ₂ x(n-2)−a ₁ y(n-1)−a ₂ y(n-2)

where b₀, b₁, b₂, a₀ and a₁ are coefficients of the second order IIRfilter, then the corresponding Z transformation function can be definedas:

${H(z)} = {\frac{Y(z)}{X(z)} = \frac{b_{0} + {b_{1}z^{- 1}} + {b_{2}z^{- 2}}}{1 + {a_{0}z^{- 1}} + {a_{1}z^{- 2}}}}$

where X(z) is the Z transformation of input x(n), and Y(z) is the Ztransformation of input y(n).

FIG. 7 gives the state transition diagram of the second order filter. Avector is defined as:

${{M(n)} = \begin{bmatrix}{m_{1}(n)} \\{m_{2}(n)}\end{bmatrix}},$

The state transition equation thereof can be expressed by:

M(n)=BM(n-1)+Cx(n)

y(n)=A ^(T) M(n-1)+b ₀ x(n)

Where

$A = \begin{bmatrix}{b_{1} - {b_{0}a_{1}}} \\{b_{2} - {b_{0}a_{2}}}\end{bmatrix}$ $B = \begin{bmatrix}{- a_{1}} & {- a_{2}} \\1 & 0\end{bmatrix}$ $C = \begin{bmatrix}1 \\0\end{bmatrix}$

The state variable M may be expressed by

${M(n)} = {{B^{n}M_{- 1}} + {\sum\limits_{k = 1}^{n}{B^{n - k}{{Cx}(k)}}}}$

The value of M(−1) determines the initial state of the filter, andaffects the transient response time of the system. If the filter is notinitialized, the default value is zero and the transient response willbe relatively long. According to the present invention, M(−1) isinitialized by the use of the input of N points; assuming the input of Npoints be X, and the output be Y,

X=[x(0) x(1) . . . x(N-1)]^(T)

y=[y(0) y(1) . . .y(N-1)]^(T)

then

Y=FM(−1)+GX

where the dimension of the vector F is N×2, the dimension of G is N·N.

$F = \begin{bmatrix}A^{T} \\{A^{T}B} \\\vdots \\{A^{T}B^{N - 1}}\end{bmatrix}$ $G = \begin{bmatrix}b_{0} & 0 & \ldots & 0 & 0 & 0 & 0 \\{A^{T}C} & b_{0} & \ldots & 0 & 0 & 0 & 0 \\{A^{T}{BC}} & {A^{T}C} & \ldots & 0 & 0 & 0 & 0 \\\vdots & \vdots & \ldots & b_{0} & 0 & 0 & 0 \\\vdots & \vdots & \ldots & {A^{T}C} & b_{0} & 0 & 0 \\\vdots & \vdots & \ldots & {A^{T}{BC}} & {A^{T}C} & b_{0} & 0 \\{A^{T}B^{N - 2}C} & {A^{T}B^{N - 3}C} & \ldots & {A^{T}B^{2}C} & {A^{T}{BC}} & {A^{T}C} & b_{0}\end{bmatrix}$

The optimal estimation of M(−1) can be obtained by the use of the methodof least mean square error.

{circumflex over (M)}(−1)=−(F ^(T) F)⁻¹ F ^(T) GX

Thus the filter can be initialized and the transient response can bereduced, and thereby the gap caused by the transient response can bereduced.

The gap filling method according to the present invention has beenverified by experiments. By using the method, taking synchronous displayof two-dimensional B type image and Doppler spectrum diagram forexample, the synchronously displayed image as shown in FIG. 8 can beobtained, which results in very good effect of continuity of the Dopplerspectrum diagram and Doppler sound.

1-15. (canceled)
 16. A method for processing Doppler signal gaps, themethod comprising: receiving Doppler signals during Doppler signalacquisition to obtain data of in-phase (I) component and quadrate (Q)component signals of the Doppler signals; receiving interruption of theDoppler signals indicating that the output of the data of the I and Qcomponent signals is interrupted in a gap period; estimating I and Qcomponent signals corresponding to the gap period based on the I and Qcomponent signals received both before and after the gap period; fillingthe interrupted I and Q component signals with the data of the estimatedI and Q component signals to generate a continuous output; generatingDoppler spectrum data based on the continuous output of the filled I andQ component signals; and generating Doppler acoustic data based on thesame continuous output of the filled I and Q component signals.
 17. Themethod of claim 16, further comprising: before estimating the I and Qcomponent signals corresponding to the gap period, filtering thereceived Doppler signals using a high pass filter.
 18. The method ofclaim 17, further comprising: determining whether the high pass filteris in a transient states or a non-transient state; if the high passfilter is determined to be in the non-transient state, buffering thehigh pass filtered I and Q component signals; and if the high passfilter is determined to be in the transient state, using the buffered Iand Q component signals for the estimation of the I and Q componentsignals corresponding to the gap period.
 19. The method of claim 17,further comprising: before generating the Doppler spectrum data and theDoppler acoustic data, wall filtering the continuous output of the dataof the filled I and Q component signals.
 20. The method of claim 16,wherein filling the interrupted I and Q component signals comprises:dividing the gap into at least a first time interval and a second timeinterval; filling the data of the interrupted Doppler signal in thefirst time interval with first filling data corresponding to the I and Qcomponent signals received before the gap period; and filling the dataof the interrupted Doppler signal in the second time interval withsecond filling data corresponding to the I and Q component signalsreceived after the gap period.
 21. The method of claim 20, wherein afirst length of time corresponding to the first filling data is shorterthan the first time interval, and wherein a second length of timecorresponding to the second filling data is shorter than the second timeinterval, the method further comprising: repeatedly outputting the firstfilling data during the first time interval; and repeatedly outputtingthe second filling data during the second time interval.
 22. The methodof claim 21, wherein the filling step further comprises: weighting thefirst filling data and the second filling data so as to smooth theDoppler data in a beginning time interval of the gap period, a pluralityof intermediate time intervals of the gap period, and an end timeinterval of the gap period.
 23. The method of claim 22, wherein theweighting comprises processing the first filling data and the secondfilling data by a window function.
 24. The method of claim 17, furthercomprising initializing the high pass filter to reduce the transientresponse time of the high pass filter, so as to shorten the gap period.25. The method of claim 24, wherein the high pass filter comprises asecond-order IIR filter.
 26. An ultrasonic scanning synchronous displaysystem, the system comprising: a transducer array to transmit ultrasonicwaves and to convert ultrasonic echoes into electrical signals; abeamformer module to receive the electrical signals from the transducerarray and to output Doppler signals during Doppler signal acquisition; aDoppler signal processing module comprising a gap filling module to:receive the Doppler signals to obtain data of in-phase (I) component andquadrate (Q) component signals of the Doppler signals; determine aninterruption of the Doppler signals, in which the output of the data ofthe I and Q component signals is interrupted in a gap period; estimate Iand Q component signals corresponding to the gap period based on the Iand Q component signals received both before and after the gap period;and fill the interrupted I and Q component signals with the data of theestimated I and Q component signals to generate a continuous output; apower spectrum estimation module to generate Doppler spectrum data basedon the continuous output of the filled I and Q component signals; and anacoustic processing module to generate Doppler acoustic data based onthe same continuous output of the filled I and Q component signals. 27.The system of claim 26, wherein the gap filling module comprises a highpass filter to filter the received Doppler signals.
 28. The system ofclaim 27, wherein the gap filling module further comprises a buffer, andwherein the gap filling module is further configured to: determinewhether the high pass filter is in a transient states or a non-transientstate; if the high pass filter is determined to be in the non-transientstate, store the high pass filtered I and Q component signals in thebuffer; and if the high pass filter is determined to be in the transientstate, use the buffered I and Q component signals for the estimation ofthe I and Q component signals corresponding to the gap period.
 29. Thesystem of claim 27, wherein the Doppler signal processing module furthercomprises a wall filter to receive the output of the gap filling moduleso as to wall filter the continuous output of the data of the filled Iand Q component signals.
 30. The system of claim 26, wherein the gapfilling module fills the interrupted I and Q component signals by:dividing the gap into at least a first time interval and a second timeinterval; filling the data of the interrupted Doppler signal in thefirst time interval with first filling data corresponding to the I and Qcomponent signals received before the gap period; and filling the dataof the interrupted Doppler signal in the second time interval withsecond filling data corresponding to the I and Q component signalsreceived after the gap period.
 31. The system of claim 30, wherein afirst length of time corresponding to the first filling data is shorterthan the first time interval, and wherein a second length of timecorresponding to the second filling data is shorter than the second timeinterval, the gap filling module further configured to: repeatedlyoutput the first filling data during the first time interval; andrepeatedly output the second filling data during the second timeinterval.
 32. The system of claim 31, wherein the gap filling module isfurther configured to: weight the first filling data and the secondfilling data so as to smooth the Doppler data in a beginning timeinterval of the gap period, a plurality of intermediate time intervalsof the gap period, and an end time interval of the gap period.
 33. Thesystem of claim 32, wherein the gap filling module is further configuredto apply a window function to the first filling data and the secondfilling data.
 34. The system of claim 37, wherein the gap filling moduleis further configured to initializing the high pass filter to reduce thetransient response time of the high pass filter, so as to shorten thegap period.
 35. The system of claim 34, wherein the high pass filtercomprises a second-order IIR filter.
 36. A system for processing Dopplersignal gaps, the system comprising: means for receiving Doppler signalsduring Doppler signal acquisition to obtain data of in-phase (I)component and quadrate (Q) component signals of the Doppler signals;means for receiving interruption of the Doppler signals indicating thatthe output of the data of the I and Q component signals is interruptedin a gap period; means for estimating I and Q component signalscorresponding to the gap period based on the I and Q component signalsreceived both before and after the gap period; means for filling theinterrupted I and Q component signals with the data of the estimated Iand Q component signals to generate a continuous output; means forgenerating Doppler spectrum data based on the continuous output of thefilled I and Q component signals; and means for generating Doppleracoustic data based on the same continuous output of the filled I and Qcomponent signals.